E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Salt Losing its Savour.
If salt has lost its savour, wherewith shall it be salted? If men fall from grace, how shall they be restored? The reference is to rock-salt, which loses its saltness if exposed to the hot sun.
A long one side of the Valley of Salt (that towards Gibul) there is a small precipice about two mens lengths, occasioned by taking away of the salt. I broke a piece off that was exposed to the sun, rain, and air; though it had the sparks and particles of salt, yet it had perfectly lost its savour. The inner part, however, retained its saltness.Maundrel, quoted by Dr. Adam Clarke.